1. Field of the Invention
This invention relates generally to switching power converters, and more particularly, to a switching converter with a startup mechanism which is useful when the load being driven is non-linear.
2. Description of the Related Art
Switching power converters can be configured using a number of different topologies, can be controlled with a wide variety of control methods, and can be used to drive different types of loads. One possible arrangement is shown in FIG. 1. In this example, a boost converter topology is employed, in which an inductor 10 is connected between an input voltage Vin and a switching element—here, a transistor 12; a diode 14 is connected to the junction of the inductor and transistor, and the converter's output voltage Vout appears on the cathode side of diode 14. Vout drives a load 16—here, a diode—connected between Vout and a node 20, and a resistor 22 having a resistance R1 is connected between node 20 and a common potential. A filter capacitor 24 is typically connected across load 16. When so arranged, the current Iload in load 16 can be controlled by controlling the voltage at node 20.
The voltage at node 20 is controlled by means of a feedback circuit. An error amplifier 26 receives a reference voltage Vref at one input and is connected to node 20 at a second input. The output of the amplifier drives a controller 28 which operates transistor 12. In operation, controller 28 operates transistor 12 as needed to make Vout the value needed to force the voltage at node 20 to Vref, thus causing Iload to be given by Vref/R1.
However, a converter arranged as shown can encounter a problem during the “startup” phase of its operation. When Vout is beginning to increase from zero, but is still below the forward voltage of diode 16, little to no current flows in resistor 22 and node 20 will be nearly equal to the common potential. With a voltage of zero at node 20, error amplifier 26 sees a large error and drives the controller to operate transistor 12 at a maximum duty ratio not controlled by feedback. This causes the average inductor current to rise rapidly, possibly to a level at which it saturates inductor 10 and possibly damages transistor 12.